How changing farming habits threaten public health

YOU can be killed by an exotic variety of diseases in India. But until recently Congo fever was not one of them. A tick-borne virus, endemic to parts of Africa, the Middle East and elsewhere, it passes easily from livestock to man, and then between humans. Horrible symptoms include fever, internal bleeding and liver failure. Some 30% of infected humans die, usually within a couple of weeks.

The authorities in Gujarat, western India, were therefore alarmed when in January a medical intern died of the disease, formally known as Crimean-Congo haemorrhagic fever. Earlier in the month it killed three others: a patient, plus a doctor and nurse who had cared for her. These appear to be the first recorded deaths in India from the illness. On February 5th doctors reported two more cases in Gujarat. The fever's arrival is a mystery.

Nobody is predicting an imminent plague. But in a region with a booming human population, a strong agricultural (notably dairy) industry and lively trade, a virus that threatens both livestock and humans is troubling. Ticks that spread Congo fever can infect, and are spread by, many wild and domestic animals, including cattle and some birds.

The example is timely. Emerging infectious diseases that affect livestock and man alike are a threat to health and prosperity. International researchers in Delhi this week met to discuss how new agricultural practices, notably in livestock, affect public health. Livestock not only pass on new diseases to other species, including man, but also help spread existing diseases to new places.

Scientists spot a new disease roughly every four months. Most are trivial but a minority, such as HIV, bird flu and SARS are grave threats. Animals seem to be the main source of new infectious disease in man: in general around 60% of human pathogens are transmissible from animals; among new diseases, the rate is about 75%.

As rural populations in India and elsewhere expand, grow richer and eat more protein, backyards where a few chickens or pigs once scratched have become densely packed smallholdings of several dozen animals. These bring owners more wealth, but also hygiene and veterinary problems. One symptom is the poor quality of meat traded in markets. A 2009 study of pork sold in Nagaland, in the north-east of India, where smallholdings have been flourishing (Christians, fond of pork, are prevalent there), found that 9% of meat contained tapeworm cysts. More than half the customers said that they had seen such cysts in their meat at some time.

Smallholdings near or in urban areas may be especially vulnerable. Humans in such places are crammed close to their animals, helping viruses to spread. Elsewhere farmers may encroach on forests or other virgin land. That exposes them and animals to once-isolated bugs. More trade in livestock helps to pass along nasties too. For humans, booming towns—those of 250,000 people or more—with big slums are ideal spots to share disease.

These worries are not new. But researchers in Delhi argue that policymakers and farmers, keen to boost food supply, give too little thought to the threats to public health. John McDermott and Delia Grace, both of the International Livestock Research Institute in Kenya, estimate that the world contains 450m smallholder farmers. Where these are most numerous, notably in Asia, they create “hot spots”, where a huge amount of germs circulate among thriving livestock and human populations, especially near cities.

Over the next couple of generations, many such smallholders will give way to larger, industrial farms, which are generally better at managing the risk of spreading plagues. But until that happens, say the researchers, small-scale farmers need to be told to cut risks, for example through cleaner farms and careful handling of carcasses. That would benefit man and beast alike.